The reason for that is that the default f(x)=cos(x) function is 1 when x is 0, and is 0 when x is pi/2. The graph shown is also 1 when x is 0, but it is 1 when x is pi/4 instead, which means that the graph is 2 times faster than the default one. This is why the answer is y=cos2x :)
